understanding soil phosphorus
TRANSCRIPT
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UNDERSTANDING SOIL PHOSPHORUS
Larry G. BundyDept. of Soil Science
University of Wisconsin
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Why is Understanding Soil P Important?
• Determine agronomic need for P• Water quality issues related to
phosphorus (P)• Restructuring of nutrient
management standard/rules to include control of P loss
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Environmental ConcernsPhosphorus is the major nutrient promoting algae and aquatic weed growth in freshwater lakes and streams.
Oxygen depletion and fish killsOdorLimits recreation and tourismQuality of drinking water drawn from surface waters.
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PHOSPHORUS AND WATER QUALITY
• Phosphorus additions to natural waters can stimulate weed and algae growth.
• Phosphorus losses from agriculture can be a major source of P entering lakes and streams.
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Sources of Phosphorus to Tainter Lake, WI
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Forms & Concentrations of Phosphorus (P) in Soils
Form Concentration (ppm)
Total 1000Soil test P (available) 20-50 Soil solution 0.01-0.30
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Phosphorus (P) Reactions in Soils
• Soluble P additions (fertilizers) react quickly to form slowly soluble compounds:
Sorbed P• Clays• Al and Fe oxidesSecondary P minerals (precipitation/dissolution)• Ca, Fe, Al phosphates
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Adsorption and Desorption of Phosphorus
• Adsorption: removal of ionic P (H2PO4-,
HPO42-) from solution by reaction with
solid phase of soil.• Solid phase: clays, oxides or
hydroxides of Fe and Al, calcium carbonates, organic matter.
• Desorption (labile P): Portion of adsorbed P available for plant uptake, extraction, or measured by soil test.
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Phosphorus (P) Loss Processes
• In surface runoff:Soluble (dissolved) PParticulate P (soil particles)
• By leachingDoes phosphorus leach?
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PHOSPHORUS (P) IN RUNOFF
• Dissolved (soluble P) (DP)• Particulate P (PP)• Total P (TP) – Includes DP and PP• Bioavailable P (BAP)
DP + part of PP
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Critical Phosphorus Concentrations for Surface Waters
Type of water
Form of P P conc. (ppm)
Lakes Soluble P 0.01Streams Total P 0.10
Lakes Total P 0.05
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Relationship between Bray P-1 (0-2 cm) and DRP in runoff.
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Influence of tillage and manure
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Spring m anure and tillage effects ontotal P load in runoff at three locations.
A rlington Lancaster Fond du Lac
Tota
l P (g
ha
-1)
0
200
400
600
800
1000
1200
1400
1600
C P, 0 t/aC P, 32 t/aN T, 0 t/aN T, 32 t/a
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Soil-specific effects on soluble P
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R e la tio n s h ip b e tw e e n S T P a n d D P c o n c e n tra tio n in ru n o ffw ith o u t s p rin g a p p lie d m a n u re .
S o il te s t P (m g k g -1)
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0
DP
in ru
noff
(mg
L -1
)
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
F o n d d u L a c(r2 = 0 .6 6 ) n = 2 8
A rlin g to n /L a n c a s te r/M a d is o n(r2 = 0 .6 5 ) n = 1 1 9
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Effect of soil extraction time on water extractable soil P concentration for two soils.
22
24
26
28
30
32
34
36
0 5 10 15 20 25 30 35 40
E xtraction tim e (shake + complete filtration), minutes
Wat
er e
xtr
act
ab
le s
oil
tes
t P (
mg
/ k
g)
Fond du LacY= 22.6 + 0.39x - 0.46x2 R2 = 0.99
LancasterY= 22.4 + 0.75x - 0.01x2 R2 = 0.98
Respective soil test P (at 60-min) = 40 and 42 mg kg-1 at Lancaster and Fond du Lac.Respective DP in runoff = 0.33 and 0.93 mg L-1 at Lancaster and Fond du Lac.
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Interpreting Soil P Tests
Soil test P, ppmCrop Optimum No responseAlfalfa 18-25 >35Corn 15-20 >30Soybean 10-15 >20
Medium and fine-textured soils, Bray P-1 test
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SOIL PHOSPHORUS BUFFERING CAPACITY
• Soil test P changes slowly with P additions or removals.
• Ave. 18 lb. P2O5/acre needed to change P test by 1 ppm
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Soil Test P Changes Slowly• Example:
–Soil P test = 100 ppm = EH–Optimum soil test = 20 ppm–Removal needed for EH to Opt. = 18
lb P2O5/acre x 80 ppm = 1440 lb P2O5
–Corn grain removes 60 lb P2O5/acre/year
–1440/60 = 24 yrs with no added P for EH change to optimum.
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Relationship between P soil test and phosphorus fertilizer recommendation
Soil Test Recommendation
Low, very low Crop removal +
Optimum Crop removal
High ½ Crop removal
Excessively High None
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Summary• Phosphorus (P) behavior in soil and
management effects on P losses are complex
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Summary• Small amounts of P can cause surface water
problems• P losses can occur as dissolved P and
particulate P
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Summary• Soils differ in effects of tillage, manure, and soil
test P on P in runoff • Excess P has accumulated in many soils• Drawdown of soil test P is a slow process
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